Electrophotography is a reprographic process comprising a charging step wherein a uniform charge is applied on the surface of a photoreceptor, an exposure step for providing a latent electrostatic image by imagewise exposure, a development step wherein the latent image is converted to a physical toner image with a developer, a transfer step wherein the toner image is transferred onto a receiving sheet, usually paper, and a fixing step wherein the toner pattern is permanently fixed to the sheet. The photoreceptor used in electrophotography consists basically of a photosensitive layer formed on an electroconductive substrate. The photosensitive layer comprises an inorganic light-sensitive material such as selenium or alloys thereof, cadmium sulfide or zinc oxide, or an organic light-sensitive material such as polyvinyl carbazole, trinitrofluorenone, bisazo pigments, phthalocyanine, pyrazoline or hydrazone. The photosensitive layer comprises one or more layers.
Photoreceptors using amorphous silicon photosensitive layers have recently been developed, and active efforts followed to improve them, as described in Japanese patent application (OPI) Nos. 78135/79 and 8634l/79. Basically, the new type of photoreceptor consists of a conductive substrate on which is deposited an amorphous silicon film by glow-discharge decomposition of silane (SiH.sub.4) gas, and the photoconductivity of the amorphous film originates from the hydrogen atoms trapped in the amorphous silicon film. The amorphous silicon photoreceptor has many advantages, such as the high surface hardness of the photosensitive layer which renders it resistant to scratching and wear, high heat resistance, high mechanical strength, and excellent spectral response properties as evidenced by high photosensitivity at wavelengths in the range of about 400 to 700 nm.
Modern laser beam printers using semiconductor lasers as light sources require electrophotographic photoreceptors which have high photosensitivity in the longer wavelength range up to approximately 800 nm. It is known that the optical band gap of an amorphous silicon photoreceptor can be decreased by doping amorphous silicon with a sufficient amount of germanium to form amorphous silicon-germanium as described in Japanese patent application (OPI) No. 190955/83. As the doping of germanium increases, the optical band gap decreases continuously from 1.7 eV (Eg of amorphous silicon) to approximately 1.1 eV (Eg of germanium). Therefore, by forming a photoconductive layer of a(amorphous)-Si.sub.1-x Ge.sub.x, photosensitive characteristics extended into the longer wavelength range can be obtained, enabling the fabrication of an electrophotographic photoreceptor having a good spectral response in the longer range up to about 800 nm.
Although amorphous silicon photoreceptors display excellent spectral response characteristics and have fairly high dark resistance, their dark resistance is not high enough to provide ideal photoreceptors. The amorphous silicon photosensitive layer undergoes a high degree of dark decay and charges of a satisfactorily high potential cannot be attained by charging a photoreceptor having this amorphous silicon photosensitive layer. If the amorphous silicon photoreceptor is subjected to the electrophotographic process comprising a charging step, an imagewise exposure step for the formation of an electrostatic latent image, and a subsequent development step, the surface charges on the photoreceptor will decay before imagewise exposure or even the charges on non-exposed areas will decay before the development step. Either factor presents difficulty in attaining the potential required for development.
Decay of the charge potential is also sensitive to ambient conditions, and a pronounced drop occurs in a hot and humid atmosphere. In addition, the charge potential will decrease gradually as a result of cyclic use of the photoreceptor. Copies obtained from a photoreceptor which has experienced a high degree of dark decay in charge potential have low image densities and are incapable of faithful halftone reproduction.